There are two large families of cast irons (alloys of iron and carbon) currently in use which differ in the appearance of the graphite particles which they contain. These families are called "flake" cast irons and "nodular" cast irons, where nodular cast irons include spheroidal cast irons as an extreme case and exclude malleable cast irons. The properties of cast irons in each of these families are well known.
Methods have recently been described for preparing cast irons which appear to be intermediate between these two types of cast iron. These new cast irons would appear to constitute a third family referred to as "vermicular" graphite cast iron or "compact vermicular" graphite cast iron by virtue of the carbon concretions disposed in the iron matrix having the appearance of small spheres which have been deformed to a greater or lesser extent into worm-like shapes.
However, although the conditions which lead to the formation of such vermicular cast irons have been known in the past, they do not appear to have found any notable industrial applications because it is difficult to obtain castings of such cast irons having a structure which is organized and reproducible.
So-called "vermicular" cast irons when used in industrial fabrication processes have a structure, both on the surface of the casting and in the bulk thereof, in which the graphite is presented in the form of a mixture of worms and nodules or of worms and flakes, or of all three together, in unstable proportions which are difficult to reproduce. This explains why they are difficult to use and why they have, heretofore, been unwanted in industrial applications.
This unreproducibility comes from the fact that when attempts are made to use such vermicular cast irons in industrial methods, they always have an unreproducible and uncontrollable proportion of nodular cast iron. The resulting industrial castings therefore have a structure both on their surface and in their bulk which cannot be controlled and which it is difficult even to reproduce.
That is why the person skilled in the art has generally considered that using vermicular cast iron industrially is very difficult and that the appearance of vermicular type cast iron in a cast iron part is an undesirable phenomenon.